1. Field of the Invention
The present invention relates to a coating material for forming a negative electrode-active material layer of a secondary battery with a nonaqueous electrolyte, a typical example of which is a lithium ion secondary battery; a negative electrode plate produced using this coating material; a method for producing the negative electrode plate; and a secondary battery with a nonaqueous electrolyte, into which the negative electrode plate is incorporated.
2. Description of the Related Art
In recent years, reduction in size and weight of electronic equipment and communication equipment has rapidly been advanced. This advance has also required reduction in size and weight of secondary batteries used as a driving power source for the equipment. For this request, there have been proposed secondary batteries with a nonaqueous electrolyte that have a high energy density and a high voltage, a typical example of which is a lithium ion secondary battery, instead of conventional alkaline batteries.
An electrode plate for a positive electrode (positive electrode plate) of a secondary battery with a nonaqueous electrolyte is produced by using, as a positive electrode-active material, a composite oxide such as lithium manganate or lithium cobaltate, dispersing or dissolving such a positive electrode-active material into a suitable wetting agent (solvent) together with a binder so as to prepare a slurry-form coating composition, and then applying the coating composition onto a collector made of metal foil so as to form a positive electrode-active material layer.
On the other hand, an electrode plate for a negative electrode (negative electrode plate) of a secondary battery with a nonaqueous electrolyte is produced by using, as a negative electrode-active material, a carbonaceous material, such as carbon, which can adsorb cations (for example, lithium ions) released from the positive electrode-active material layer at the time of charging, dispersing or dissolving such a negative electrode-active material into a suitable wetting agent (solvent) together with a binder so as to prepare a slurry-form coating composition, and then applying the coating composition onto a collector made of metal foil so as to form a negative electrode-active material layer.
A terminal for taking out electric current is then fitted to each of the positive electrode plate and the negative electrode plate, and subsequently a separator for preventing a short circuit is sandwiched between the two electrode plates, and wound thereon. Thereafter, the resultant is put into a container filled with a nonaqueous electrolyte solution and then the container is sealed, thereby fabricating a secondary battery.
Regarding the binder for preparing the active material coating solution in the above-mentioned coating type electrode plates, the following are required: 1) The binder is chemically stable against the nonaqueous electrolyte; 2) The binder is not eluted into the electrolyte; and, 3) The binder is dissolved in some solvent, and the resultant solution can be applied to a substrate so as to give a thin layer.
Furthermore, for the applied and dried active material layer, the following are required: 1) The layer has flexibility such that it does not peel, fall away or crack in the battery-fabricating process; and, 2) The layer is superior in adhesiveness to the collector.
In the case where a coating type electrode plate is produced, an active material coating solution is usually applied onto a collector to form a coating layer and then pressing is performed in order to improve the adhesiveness of the active material layer to the collector, the density thereof and the uniformity thereof. However, when press-working such as roll-press or sheet-press is performed, the coating layer may be adsorbed on the press face, that is, a part of the coating layer may be adhered to the press face so as to be exfoliated off from the collector. As a result, an inferior product may be produced. In particular, in the case where a binder with a high tackiness is used in order to give the active material layer exhibiting a sufficient adhesiveness to the collector while reducing the amount of the binder so as not to lower battery performance, the frequency that the active material layer is exfoliated by the press face at the time of the press-working increases. Thus, the yield of batteries deteriorates.